Harmonic analysis of AC-bus voltage in converter station based on time–frequency transformation

The disturbance of the AC system has become one of the main causes of commutation failure in the high voltage direct current (HVDC) transmission system. Here, the time–frequency transformation method is used to analyse the AC-bus voltage disturbance from the perspective of the harmonic content of the AC power network. The time–frequency transformation for AC-bus voltage disturbance is difficult due to its characteristics of time variability and time fixity. With comparative analysis of various transformation methods, the method of combining the wavelet packet transform with the short-time Fourier transform can meet the requirement of time–frequency analysis of the AC voltage disturbance very well, and this method was used to analyse the fault recording data of Fengjing Convert Station when commutation failure occurred in recent years. The result shows that the AC-bus voltage mainly contained third and fifth harmonics, and there was a certain amount of even harmonics, and the low-order harmonics would be more in the case of a remote fault. In addition, there were more harmonics in the voltage when faults occurred in the 500 kV system, while there were more low-order harmonics when faults occurred in the other voltage class systems